Acoustics often has been regarded as a science dealing only with hearing conditions in auditoriums, but there are other aspects of the environment in which it plays an important role. Every office building, airport terminal, hospital, and even the private house has sound and noise-control problems. If these problems are not recognized and dealt with during the design of the structure, they must be corrected later.
A sound source produces pressure variations in the air, compressions and rarefactions moving outward from the source in a progressive wave at a speed of about 1100 feet per second. In free space, the energy intensity in the wave decreases as the distance from the source increases. Sound waves indoors are reflected many times by the enclosing surfaces, the intensity remaining more or less uniform throughout a room. The sound wave carries energy from the source to the ear or other receiver.
The amount of sound energy that reaches the listener depends on the paths by which the sound has traveled, on the distance from the source, the power of the source, and the nature of such intervening barriers as walls, doors, windows, and sound-absorbing ducts. Qualities that characterize a desirable acoustic environment vary widely, depending on how the space is to be used, how particular the users are, and how the specific space involved relates to other parts of the building. The acoustic character of a space is as important as the amount of sound generated in it or transmitted into it. If a room is finished in hard, sound-reflecting materials, sounds in the room not only persist but seem to come from all directions. There are optimum ranges for the reverberant character of occupied spaces.
When a sound wave strikes a sizable surface, such as a wall or ceiling, part of it is reflected, part of it is absorbed, and part of it may be transmitted to some adjoining space. The relative magnitudes of the three parts of the original sound are determined by the physical properties of the surface. A hard-surfaced, dense plaster reflects most of the sound that strikes it, while a soft surface such as a heavy carpet reflects little sound. Absorption and reflection are important only in connection with the space in which the sound originates. Useful sound absorption is provided by porous material such as carpets, draperies, glass-fiber blankets, clothing, and specially-made sound-absorbing materials. An essential property of a sound-absorbing material is that it have a porous structure into which the molecules in the air carrying sound energy can dissipate that energy in the form of heat. Sound energy so dissipated is never recovered as sound. When sound-absorbing materials are placed within a room, the reflection of energy is greatly reduced and the sound dies away rapidly.
Good hearing conditions for an audience listening to speech or music, whether in an auditorium, courtroom, church, theater, or living room, depend on four basic conditions. The space must be quiet, the sounds to be heard must have adequate loudness, there must be a good dispersion of sound, and the sounds must be properly blended, with adequate separation for good articulation of music or speech. This latter quality is particularly important with regard to the present invention.
The blending of sounds is produced by a process that is called reverberation. The reverberation time in a room is arbitrarily defined as the time required for the sound to decrease to one-millionth of its original intensity after the source is cut off. Reverberation time is determined by the volume of the room and by the sound-absorbing properties of the finishes, furnishings, and people in the room. The greater the volume, the greater the reverberation time. The greater the area of sound-absorbing finishes, however, the less the reverberation time. In many theaters and auditoriums the audience is a principal absorber of sound. For this reason fabric-upholstered seats that absorb sound in the same way as the audience does are employed to minimize the variation in the reverberation time with varying audience size.
In general, a relatively long reverberation time is needed for a sufficient blending of musical sounds but a shorter one is needed for the proper understanding of speech. A matter not well understood until recently has been the importance of balancing very carefully the amount of early sound (original signal plus early reflection) with the energy that reaches the ear in the reverberant field (after many reflections about the room). If there is too much early sound, the effect is too articulate and "dry". On the other hand, if there is too much reverberant energy, the effect is fuzzy, or "wet". The optimum ratio between the early and the reverberant energy varies with the preferences of the users of the space.
Wallace Clement Sabine, in the early 20th century, appears to have been the first person to recognize that the performance of an auditorium can be quantified. Sabine defined the reverberation time of a space as the time required for the sound energy in an enclosure to decrease by a factor of one million and gave a formula for the reverberation time as: ##EQU1## Where V is the volume of the space, .alpha. the average absorption coefficient, and S the surface area (all in English units). Sabine recognized that the optimum reverberation time for a room increases with its volume and depends upon frequency, as well as upon the kind of sound that one is trying to enhance.
During the first half of the 20th century, other investigators improved both the physical basis of architectural acoustics and the psychological basis, mostly empirical, of optimum room characteristics. Among these investigators the names of Morse, Beranek, Knudsen and Harris are some of the best known, although many other people made important contributions.
In the acoustical design of a room, it is common practice to try to achieve a desired value of reverberation time at six frequencies at octave intervals; namely, 125, 250, 500, 1,000, 2,000, and 4,000 Hertz (Hz). This frequency range covers all of the range of song and speech "fundamentals" and most of the range of orchestral instruments, though not all. There are, of course, other considerations in acoustical design, one of these being the arrangement of surfaces in order to yield desired dispersion of the sound field.
It is a fact of physics that velocity (of the transmitting medium) in a sound wave is directly proportional to frequency. It follows that any substance which absorbs sound will tend to absorb high frequencies more than low frequencies. This is found to be so by measurement. However, the principle of resonance in physics can be used to increase the velocity in a sound wave at or near a resonant frequency and thus to increase absorption at low frequencies by design of suitable resonators.
It is an object of the present invention to provide a cost-effective method and apparatus of acoustically designing an enclosed environment.
It is another object of the present invention to provide a tunable, resonant acoustic panel.
It is still another object and advantage of the present invention to provide an acoustic panel that is simple to install.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended Claims.